Abstract

Background The psychosis phenotype is generally thought of as a
categorical entity. However, there is increasing evidence that psychosis
exists in the population as a continuum of severity rather than an all-or-none
phenomenon.

Aims To investigate the prevalence and correlates of self-reported
psychotic symptoms using data from the 2000 British National Survey of
Psychiatric Morbidity.

Method A total of 8580 respondents aged 16–74 years were
interviewed. Questions covered mental health, physical health, substance use,
life events and socio-demographic variables. The Psychosis Screening
Questionnaire (PSQ) was used to identify psychotic symptoms.

Conclusions Self-reported psychotic symptoms are less common in this
study than reported elsewhere, because of the measure used. These symptoms
have demographic and clinical correlates similar to clinical psychosis.

In recent years there have been suggestions that psychosis exists in the
general population as a continuous phenotype rather than as an all-or-none
phenomenon (van Os et al,
2000). The existence of a psychosis continuum has been found in
several large-scale community surveys. In the US National Comorbidity Survey,
28% of individuals endorsed psychosis-screening questions although the rate of
clinician-defined psychosis was only 0.7%
(Kendler et al,
1996). In the Dunedin birth cohort, 25% of the sample at age 26
years reported at least one delusional or hallucinatory experience that was
unrelated to drug use or physical illness, but only 3.7% actually fulfilled
criteria for schizophreniform disorder
(Poulton et al,
2000). Data on psychotic symptoms were collected as part of the
Dutch NEMESIS study (van Os et
al, 2000). This representative general population sample of
7076 men and women was interviewed using the Composite International
Diagnostic Interview (CIDI), which contains 17 core positive psychosis items;
17.5% of the sample scored at least one on any type of positive psychosis
rating but only 2.1% received a DSM–III–R
(American Psychiatric Association,
1987) diagnosis of non-affective psychosis. Together, these
findings suggest that only a small part of the total phenotypic continuum of
psychosis is represented by clinically verified and defined cases. Psychotic
symptoms in the general population are associated with certain risk factors
(e.g. urban residence or younger age group;
Verdoux et al, 1998;
van Os et al, 2000,
2001). Furthermore, the risk
factors for psychotic symptoms mirror those for clinical psychosis, supporting
the continuum hypothesis. Individual psychotic symptoms also seem to have
specific correlates, suggesting different risk factors for these symptoms. For
example, it has been reported that paranoia is associated with experience of
victimisation (Janssen et al,
2003) and that hallucinatory experiences are more common among
people of Caribbean origin living in Britain
(Johns et al, 2002).
We used data from a large cross-sectional survey of the British population to
examine the distribution and correlates of self-reported psychotic symptoms.
We also examined whether there were any specific demographic and clinical
correlates of paranoid thoughts and hallucinatory experiences.

METHOD

Sample

The data were obtained from the second National Survey of Psychiatric
Morbidity in Great Britain, conducted in 2000 by the Office for National
Statistics (ONS). The survey examined the prevalence of mental health problems
among adults aged 16–74 years living in private households in Great
Britain. It covered mental health, physical health, drug and alcohol use, life
events, service use and socio-demographic variables. The sample was drawn from
the small-user Postcode Address File using a two-stage approach. Initially,
postcode sectors were stratified on the basis of socio-economic status within
region and 438 sectors were chosen with a probability proportional to size.
Then, within each sector, 36 addresses were randomly selected for inclusion in
the survey. One adult aged 16–74 years was interviewed in each
household. The survey used a two-phase design. First-phase interviews were
carried out by interviewers from the ONS field force. Personal interviews were
carried out with 8580 adults (a response rate of 67%). Individuals who scored
positively on one or more psychosis criteria were part of a sub-sample who had
a follow-up second-phase interview by a clinician. Details of the survey
methods can be found in Singleton et al
(2001).

Assessment of psychotic symptoms

In the initial interview, the Psychosis Screening Questionnaire (PSQ;
Bebbington & Nayani, 1995)
was used to assess psychotic symptoms in the past year. The PSQ has five probe
questions (plus secondary questions) enquiring about mania, thought insertion,
paranoia, strange experiences and hallucinations (see Appendix). Respondents
were asked all the items from the PSQ without the usual procedure of cutting
off after a section was answered positively.

For the current analyses, we selected individuals who endorsed one or more
psychotic symptom (initial probe plus secondary questions) on the PSQ. Because
we wanted to examine psychotic experiences in non-clinical subjects, we first
excluded those people with definite or probable psychosis (defined below).

Assessment of psychosis

Four criteria from the first-phase interview were considered likely to be
indicative of psychosis: a self-reported diagnosis or symptoms suggestive of
psychotic disorder; taking anti-psychotic medication; a history of admission
to a psychiatric hospital or ward; a positive response to question 5a of the
PSQ (auditory hallucinations). Respondents who met one or more of these
psychosis screening criteria were selected for a follow-up interview using the
Schedules for Clinical Assessment in Neuropsychiatry (SCAN;
World Health Organization,
1992a). Algorithms then were used to classify respondents
into ICD–10 (World Health
Organization, 1992b) psychosis categories. Some people
selected for a second-phase interview could not be contacted or refused a
second interview; in these cases, an assessment of ‘probable
psychosis’ was assigned to those who scored positively on two or more of
these psychosis criteria.

Life events. Respondents were asked whether they had experienced any of 11
stressful life events taken from the List of Threatening Experiences (life
events with long-term threat; Brugha et
al, 1985) in the 6 months prior to interview. The life events
are: serious illness, injury or assault to you; serious illness, injury or
assault to a close relative; death of a close relative; death of a close
friend/other relative; separation or divorce; serious problem with a close
friend, neighbour or relative; made redundant or sacked; unemployed/seeking
work for more than 1 month; major financial crisis; problem with police and
court appearance; something valued that is lost or stolen.

Victimisation events. Respondents were asked whether they had ever
experienced any of the following: bullying; violence at work; violence in the
home; sexual abuse; being expelled from school; running away from home; being
homeless.

Analyses

The data were analysed using the Statistical Package for the Social
Sciences (version 11.0 for Windows). Binary logistic regression analyses were
used to ascertain which factors were associated with the presence of psychotic
symptoms. Associations were expressed as odds ratios (ORs). Sixty people
(0.7%) with probable psychosis were excluded from the analyses, 27 of whom met
the criteria for functional psychosis following a SCAN interview.

We examined the factors associated with the presence of any psychotic
symptom (endorsement of initial plus secondary questions on one or more items
of the PSQ). First, the predictor variables were entered individually to
obtain unadjusted odds ratios. Age was collapsed into three age bands
(16–34, 35–54, 55–74). Information about ethnic origin was
divided into four groups: White, Black, South Asian and Other. Area of
residence was divided into urban and rural. Educational qualifications covered
three groups: none, GCSE level, A-level or above. Verbal IQ was estimated from
respondents’ scores on the National Adult Reading Test (NART;
Nelson, 1982). Alcohol
dependence was classified as present or absent. Drug use was any drug used in
the last month (yes/no); and drug dependence was classified as no dependence,
dependent on cannabis only or dependent on other drug. Experience of life
events and victimisation events was classified dichotomously (yes/no). Second,
all the significant variables were entered together to obtain the relative
odds of psychotic symptoms controlling for interrelationships between these
variables.

We examined whether specific factors were associated with the presence of
either paranoid thoughts or hallucinatory experiences. These items were chosen
because previous studies have suggested that they are associated with
particular risk factors (Johns et
al, 2002; Janssen et
al, 2003). The response variables selected were ‘Have
there been times when you felt that people were deliberately acting to harm
you or your interests?’ and ‘Have there been times when you heard
or saw things that other people couldn’t?’ In order to examine
specific risk factors, the analyses for each variable included individuals who
had endorsed only that item and no other PSQ items.

RESULTS

Frequency of psychotic symptoms

After excluding people with probable psychosis (n=60), data were
available for 8520 individuals. Of this remaining sample, 5.5% reported one or
more psychotic symptom as measured by the PSQ. For each item, more people
endorsed the initial probe question than the secondary question(s)
(Table 1). Thus, for paranoia,
9.1% endorsed the question ‘Have there been times when you felt that
people were deliberately acting to harm you or your interests?’ whereas
only 1.5% endorsed ‘Have there been times when you felt that a group of
people were plotting to cause you serious harm or injury?’ Similarly for
hallucinations, 4.2% of the sample said that there had been times when they
heard or saw things that other people could not, but only 0.7% reported
hearing voices saying quite a few words or sentences when there was no-one
around that might account for it.

Socio-demographic distribution of self-reported psychotic
symptoms

Frequencies of the variables examined for associations with psychotic
symptoms

Any psychotic symptom

In the initial unadjusted analysis, the following variables were associated
with the presence of any self-reported psychotic symptom: drug dependence, the
presence of neurotic disorder, drug use, victimisation events, alcohol
dependence, recent stressful life events, non-White ethnic group, younger age,
lower IQ, fewer educational qualifications and urban residence
(Table 3). Gender was not a
significant predictor. In the final model of adjusted odds ratios, neurotic
disorder and drug dependence were the variables most strongly associated with
psychotic symptoms. Individuals were 3.5 times as likely (95% CI
2.9–4.5) to experience one or more psychotic symptoms if they scored
above 12 on the CIS–R, were almost three times as likely (95% CI
2.0–4.4) if they were dependent on cannabis and were just under 2.5
times as likely (95% CI 1.3–3.9) if they were dependent on any other
drug (with or without cannabis). Experience of victimisation and alcohol
dependence were also strongly associated with psychotic symptoms (OR=2.0, 95%
CI 1.7–2.6; OR=1.8, 95% CI 1.3–2.4, respectively). After
controlling for interrelationships between all the variables, young age,
non-White ethnic group, urban residence and recent drug use were no longer
associated significantly with psychotic symptoms.

DISCUSSION

This study examined the distribution and correlates of self-reported
psychotic symptoms in the British population. Data were available for a large
representative sample of the general population, and included a wealth of
information on symptoms, socio-demographic factors, substance use and life
events. As with any cross-sectional survey, this study lacks information on
temporal relationships, and therefore it is possible to report only
associations between variables.

Prevalence of psychotic symptoms in the sample

The annual prevalence of psychotic symptoms, in the absence of psychotic
disorder, was 5.5%. This refers to the percentage of people who endorsed one
or more items on the PSQ, including the secondary questions for the item (i.e.
the more ‘psychotic’ experiences). As shown in
Table 1, the prevalence was
higher for the ‘less psychotic’ responses, consistent with the
existence of a continuum of psychotic phenomena in the general population. The
reported figure is lower than the rates of psychotic symptoms reported by
other epidemiological studies (17.5%,
Poulton et al, 2000;
25%, van Os et al,
2000). It is likely that variation in prevalence rates across
studies is partly a consequence of the different instruments used (number and
type of questions asked). The PSQ is a brief measure that assessed only five
psychotic symptoms. One would expect higher prevalence rates with a more
comprehensive measure such as the CIDI, which contains 17 psychotic symptom
items. Also, as we found within the PSQ, questions probing for ‘less
psychotic’ experiences are likely to be endorsed more frequently. In
addition to differences in the measures, most epidemiological studies have
assessed the lifetime prevalence of psychotic symptoms. This will be much
greater than the annual prevalence, as measured by the PSQ.

Factors associated with any psychotic symptom

There were associations between psychotic symptoms and neurotic disorder,
drug dependence, alcohol dependence, victimisation experiences, recent
stressful life events, lower IQ and fewer educational qualifications. Younger
age group, non-White ethnic group and urban residence no longer significantly
predicted psychotic symptoms after controlling for interrelationships between
predictor variables. In terms of drug dependence, the relationship between
cannabis dependence and psychotic symptoms was the strongest and also may have
contributed to the association between other drug dependence and psychotic
symptoms.

The results from this sample replicate previous findings concerning risk
factors associated with psychotic symptoms. Van Os et al
(2000) reported an association
between neurotic symptoms and all types of psychosis ratings, from ‘not
clinically relevant’ symptoms to clinical psychosis. From our results,
it is not possible to know whether neurotic disorder is associated with
increased risk of developing psychotic symptoms, or whether it is a
consequence of experiencing psychotic symptoms. In a study of adult primary
care patients, Olfson et al
(2002) found that psychotic
symptoms were associated with anxiety, depression and alcohol use disorder,
and suggested that the latter were all clinical consequences. On the other
hand, neurotic symptoms have been reported in excess in those children who
later develop psychosis (Jones et
al, 1994; Cannon et
al, 2002), and have been identified as part of the initial
prodrome in psychosis (Yung & McGorry,
1996). Longitudinal studies have found that adolescent males with
neurotic disorders are more likely to develop schizophrenia years later
(Weiser et al, 2001),
and that neuroticism (which is related to anxiety proneness) increases the
risk for subsequent onset of psychotic symptoms
(Krabbendam et al,
2002). In a recent review, Freeman & Garety
(2003) argue that the frequent
occurrence of emotional disorder prior to and accompanying psychosis suggests
that neurosis contributes to the development of the positive symptoms of
psychosis.

Similarly, we cannot determine the direction of the relationship between
cannabis dependence and psychotic symptoms from these data. However, a number
of cohort studies have shown that consumption of cannabis is a risk factor for
later psychosis (e.g. Andreasson et
al, 1987; van Os et
al, 2002; Zammit et
al, 2002). Thus, Arsenault et al (2002) found that
cannabis use in adolescence increased the risk of experiencing schizophrenia
symptoms in adulthood, indicating a causal link. Furthermore, this risk was
specific to cannabis use, as opposed to the use of other drugs. The
association between alcohol dependence and psychotic symptoms may be related
to the occurrence of withdrawal symptoms.

Consistent with previous studies (e.g.
van Os et al, 2000),
lower educational achievement was associated with self-reported psychotic
symptoms. The association with potentially threatening life events and
victimisation events also corresponds with previously reported risk factors
for psychosis. Using the same National Survey data, Bebbington et al
(2004) found a high prevalence
of reported victimisation among people with psychosis, greater than that found
among people with neurotic disorder or drug and alcohol dependence. The
association between non-White ethnic group and psychotic symptoms in this
study was no longer significant after controlling for other factors, including
victimisation and stressful life events. Although it has been reported that
Black and ethnic minority patients with psychosis do not experience more life
events than do White British patients, they do perceive these events as more
threatening (Gilvarry et al,
1999).

Factors associated with paranoid thoughts

Paranoid thoughts were associated with neurotic symptoms, victimisation
experience(s), younger age, alcohol dependence, stressful life events in the
past 6 months, average IQ and male gender. The relationships between paranoia
and victimisation and stressful life events are consistent with cognitive
psychological theories about the development and maintenance of psychotic
symptoms (Garety et al,
2001; Freeman et al,
2002). Thus, experiences of victimisation may lead individuals to
believe that they are vulnerable and to view other people and the world as
hostile and threatening; and stressful events may then trigger symptoms.
Janssen et al (2003)
found that perceived discrimination was associated longitudinally with onset
of delusional ideation. Unfortunately, it is not possible from these data to
determine the precise temporal relationships between victimisation, life
events and paranoia. It could be that subjects with paranoid thoughts have a
biased recall for these experiences, or that the supposedly paranoid thoughts
are actually real, and that people are trying to harm them. Neurotic symptoms
were strongly associated with paranoid thoughts in this sample. Again, this
result is consistent with cognitive models of persecutory delusions
(Freeman et al, 2002),
in which anxiety and depression (particularly anxiety) are thought to play a
central role in the formation of paranoid beliefs.

Factors associated with hallucinatory experiences

Neurotic disorder, victimisation experiences, average and below-average IQ,
alcohol dependence and female gender were associated with hallucinatory
experiences. There was a trend for an association between hallucinations and
Black ethnic group, which replicates the findings of Johns et al
(2002) from an earlier UK
national survey. In that study, the prevalence of hallucinations assessed by
the PSQ in a sample of people of Caribbean origin living in Britain was 2.5
times higher than that in a White sample, although no statistical comparison
was reported. The finding that women were more likely to report hallucinatory
experiences is consistent with the higher rates of hallucinations in females
found in previous studies (Tien,
1991). As with paranoia, the association with victimisation is
consistent with psychological theories of hallucinations, particularly the
link between exposure to trauma and the development of hallucinations
(Romme & Escher,
1989).

Further research

This study found that self-reported psychotic experiences in the general
population were associated with risk factors similar to those commonly
reported for clinical psychosis. Using the interview data from both phases of
the survey, we now intend to compare the correlates of self-reported psychotic
symptoms and clinical psychotic disorder in this sample.